The catalytic hydrogenation of furfural can lead to numerous derivatives, including furfuryl alcohol (“FA”), 2-methylfuran (“2-MF”) and 2-methyltetrahydrofuran (“2-MTHF”). Those skilled in the art recognize that there is a substantial commercial market for processes that efficiently produce furfural derivatives, such as FA, 2-MF and 2-MTHF, because they are marketable as specialty chemical products. For example, 2-MTHF is useful as a component of certain alternative fuel compositions that are cleaner burning than present liquid fuels. Currently, there are only a few commercial-scale production facilities for 2-MF and 2-MTHF because there is a lack of economically-viable methods for their preparation.
Some of the reaction pathways for the catalytic hydrogenation of furfural to various furfural derivatives are illustrated below. 
Since the early part of the twentieth century, researchers have investigated the mechanisms of individual reaction pathways that start with the catalytic hydrogenation of furfural.
Several of the previously studied reaction pathways are illustrated below, namely, pathways for the production of FA, 2-MF and 2-MTHF. 
Although both liquid-phase and vapor-phase hydrogenation of furfural are possible, the majority of the previous studies were focused on developing processes for the liquid-phase hydrogenation of furfural. Although liquid-phase hydrogenation to produce 2-MTHF is theoretically possible, no literature was found which reported a successful use of a liquid-phase hydrogenation process to produce 2-MTHF. Sometimes 2-MF was created, but often as a by-product, FA being one of the primary products of the liquid-phase hydrogenation. The liquid-phase hydrogenations were most often run batch-wise in the laboratory and produced relatively small amounts of product, in part due to inherent defects associated with the custom-made catalysts, such as prohibitive preparation costs and inconsistent preparations, that led to inconsistent yield or purity of the products formed.
In addition, the liquid-phase hydrogenations were often run at greater-than-atmospheric pressure and used specialized equipment capable of operating at elevated pressures. As a result of these and other limitations, liquid-phase hydrogenations were not easily scaled up beyond the laboratory bench level, were difficult to control and required large amounts of energy to complete, thus reducing the overall efficiency and yield of liquid-phase hydrogenation processes. The chart below summarizes some general differences between liquid-phase hydrogenations and vapor-phase hydrogenations.
Liquid-phaseVapor-phaseHydrogenationHydrogenationPressurehighlow or atmosphericScaleabledifficulteasierProcessbatch-typebatch-type or continuousProcess controldifficulteasierCatalyst recoveryrequirednot requiredReactorhigh-pressure vesselpacked-column, tray towerF to 2-MF Yieldslowquantitative2-MF to 2-MTHFnot known in manymoderate to highYieldsinstancesProductscomplex mixturefewer, easily separated byproducts
The following references reported the results of research conducted in the area of vapor-phase, catalytic hydrogenation of furfural.
U.S. Pat. No. 1,739,919, issued Dec. 17, 1929 to Ricard and Guinot, disclosed a process for “the manufacture of furfuryl alcohol and methylfurane [sic].” The furfural feedstock was vaporized and mixed in a vaporizer with a current of hot hydrogen gas. The furfural vapor and hydrogen gas mixture was delivered into a “catalyzing furnace” packed with a reduced copper oxide catalyst. The custom-made catalyst was prepared in the laboratory by “precipitation and calcination” and a “backing substance” such as asbestos, pumice stone, silica or kaolin, was used to provide support for the catalyst during the reaction. The vapor-phase hydrogenation was run with fresh catalyst at atmospheric pressure and at a temperature of 140° C. The vapor issuing from the “catalyzing furnace” was passed through a condenser. The condensed liquids were a mixture of methylfuran, furfural, furfuryl alcohol and water. The methylfuran was removed from the mixture by simple distillation. The proportion of methylfuran was 10 to 20 percent of the furfuryl alcohol formed.”
U.S. Pat. No. 2,273,484, issued Feb. 17, 1942 to Guinot, disclosed a process “for manufacturing synthetic rubber from furfurol [sic].” One aspect of the invention was using a vapor-phase hydrogenation to convert furfural to methylfuran using a copper catalyst. The methylfuran obtained from this hydrogenation was then passed as vapor over a reduced nickel catalyst at a temperature of 120° C. to produce a mixture of products, including 85 percent MTHF. No other details regarding the conditions of the processes were disclosed.
U.S. Pat. No. 2,456,187, issued Dec. 14, 1948 to Guinot, disclosed a process for catalytically hydrogenating organic substances. In one example, furfural was hydrogenated using “pure copper obtained by reducing oxide thereof.” The custom-made catalyst was made in the laboratory of copper oxide precipitated on an inert carrier such as asbestos, silica or pumice. The catalyst and method of using the same were designed in an attempt to prevent problems with undesired local elevations of temperature during the reduction of the catalyst or during the hydrogenation. Vaporized furfural was mixed with hydrogen and delivered into an apparatus containing bundles of tubes packed with tablets of the catalyst. The hydrogenation was performed at a temperature of 170° C. and the product mixture was condensed, consisting mostly of furfuryl alcohol (60 percent) as well as some methylfuran (40 percent). In addition, methylfuran was produced by hydrogenation of furfuryl alcohol at a temperature of 170° C.
In 1948, Burnette et al. studied twenty-three custom-made catalysts prepared in the laboratory for use in the production of 2-MF by vapor-phase hydrogenation of furfural. See Burnette et al., “Production of 2-Methylfuran by Vapor-Phase Hydrogenation of Furfural,” I&EC, 40 (3):502-505 (1948). The paper noted that “the primary object of almost all previous work on the hydrogenation of furfural has been the production of furfuryl or tetrahydrofurfuryl alcohol.” Id. at 502. The best laboratory-scale (15 to 100 grams of furfural) yields of 2-MF (99.5 percent) resulted when a copper chromite catalyst (dispersed on activated charcoal) was used and a temperature of 225° C. was maintained during the hydrogenation. However, the yields (grams of 2-MF produced/grams of furfural hydrogenated) of 2-MF produced as a result of a larger (500 grams of furfural) work-up of this process were much lower, varying from 67-80 percent. Problems noted by the researchers during the scale-up of the process included the almost total deactivation of the catalyst after 7 days of nominal usage and a 20° C. to 30° C. temperature variation between the center of the catalyst bed and the reactor wall. These problems caused the overall efficiency and economic viability of the reaction to be reduced.
Later, this same laboratory reported using the same type catalyst to produce high yields of furfuryl alcohol. After careful consideration of various factors impacting the reaction, the researchers concluded that the temperature variable was the most critical. In these later experiments, furfural was vaporized and mixed with hydrogen (in a molar ratio of hydrogen to furfural in excess of 20:1) and then delivered into a reactor tube heated to between 130° C. to 135° C. and containing a copper chromite catalyst supported on charcoal. They catalyst bed temperature was reported in some experiments to be as low as 112° C. and in other experiments as high as 192° C. In all experiments, the primary product was furfuryl alcohol (recovered in yields widely ranging from 26 to 93 percent) and 2-MF was collected as a by-product. See Brown et al., “Vapor Phase Hydrogenation of Furfural to Furfuryl Alcohol,” I&EC, 40 (3):1382-1385 (1949).
U.S. Pat. No. 3,021,342, issued Feb. 13, 1962 to Manly disclosed hydrogenation of pyrans and furans, including the vapor-phase hydrogenation of methylfuran to produce methyltetrahydrofuran, preferably using a custom-made, reduced nickel hydrate catalyst. The methylfuran, containing pyridine as a selective poison for the catalyst, was vaporized and blended into a stream of preheated hydrogen gas. The resulting methylfuran/hydrogen vapor mixture was passed through a catalyst column and the reaction products were collected and condensed. The hydrogenation was run at a preferred temperature range of 60° C. to 65° C. The reaction occurred at about or in slight excess of atmospheric pressure. Conversion of the methylfuran was almost 100 percent and the product analyzed about 98.4 percent, by weight methyltetrahydrofuran.
These results reported by researchers in the area of vapor-phase, catalytic hydrogenation of furfural demonstrate that significant effort was required to find appropriate catalysts and reaction conditions that direct the vapor-phase, catalytic hydrogenation of furfural along a particular reaction pathway towards the production of a desired furfural hydrogenation derivative. Those skilled in the art recognize that an even greater amount of experimentation would be required to find particular combinations of catalysts and reaction conditions necessary to produce furfural hydrogenation derivatives in both high yields and with high purity in an economic and commercially-viable manner.
Prior attempts to develop processes for the vapor-phase, catalytic hydrogenation of furfural, including those described above, had one or more of the following disadvantages: low yields and/or low purity of 2-MF and/or 2-MTHF, low conversion of furfural to 2-MF and 2-MTHF and/or of 2-MF to 2-MTHF, production of undesirable by-products, production of by-products that were difficult to separate from the desired products, high process costs due to the low efficiency of the catalysts and/or due to the short life of the catalysts and an inability to sufficiently control the temperature during the hydrogenations.
In addition, because the catalysts used in these prior art attempts were prepared in small quantities in the individual laboratories, the results obtained using such catalysts were inconsistent and the possibility of commercial-scale processes was precluded. The custom, small-scale preparation of such catalysts often led to inconsistent catalyst quality and lowered yields and purity of the desired products. Thus, there is a continuing need for commercially-viable, high-yield furfural vapor-phase hydrogenation processes which efficiently produce pure products, including furfural derivatives, such as 2-MF and 2-MTHF, using commercially available, low-cost catalysts.